H. Tian, E. E. DeLuca, S. R. Cranmer, et al., Science 346, 1255711 (2014) Prevalence of Small-scale Jets from the Networks of the Solar Transition Region and Chromosphere H. Tian,1* E. E. DeLuca,1 S. R. Cranmer,1 B. De Pontieu,2 H. Peter,3 J. Martínez-Sykora,2,4 L. Golub,1 S. McKillop,1 K. K. Reeves,1 M. P. Miralles,1 P. McCauley,1 S. Saar,1 P. Testa,1 M. Weber,1 N. Murphy,1 J. Lemen,2 A. Title,2 P. Boerner,2 N. Hurlburt,2 T. D. Tarbell,2 J. P. Wuelser,2 L. Kleint,2,4 C. Kankelborg,5 S. Jaeggli,5 M. Carlsson,6 V. Hansteen,6 S. W. McIntosh7 1Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA. 2Lockheed Martin Solar and Astrophysics Laboratory, 3251 Hanover St., Org. A021S, Bldg. 252, Palo Alto, CA 94304, USA. 3Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany. 4Bay Area Environmental Research Institute, 596 1st St West, Sonoma, CA 95476, USA. 5Department of Physics, Montana State University, Bozeman, P.O. Box 173840, Bozeman, MT 59717, USA. 6Institute of Theoretical Astrophysics, University of Oslo, P.O. Box 1029, Blindern, NO-0315 Oslo, Norway. 7High Altitude Observatory, National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307, USA. *Correspondence to: [email protected] Abstract: As the interface between the Sun’s photosphere and corona, the chromosphere and transition region play a key role in the formation and acceleration of the solar wind. Observations from the Interface Region Imaging Spectrograph reveal the prevalence of intermittent small-scale jets with speeds of 80-250 km s-1 from the narrow bright network lanes of this interface region. These jets have lifetimes of 20-80 seconds and widths of ≤300 km. They originate from small-scale bright regions, often preceded by footpoint brightenings and accompanied by transverse waves with ~20 km s-1 amplitudes. Many jets reach temperatures of at least ~105 K and constitute an important element of the transition region structures. They are likely an intermittent but persistent source of mass and energy for the solar wind. Main Text: The Sun continuously emits ionized particles into interplanetary space in the form of the solar wind. A challenging investigation has now carried on for almost 50 years to understand where the solar wind originates and how it is accelerated (1, 2). Dark regions in coronal images indicate the coronal holes that are the commonly accepted large-scale source regions of the high-speed solar wind. However, identifying precise origin sites within coronal holes requires high- resolution observations of the chromosphere and transition region (TR), a complex interface between the relatively cool photosphere (~6×103 K) and hot corona (106 K). The mass and energy that ultimately feeds the solar wind must pass through this region. 1 The dominant emission features in this interface region are the network structures that appear as narrow bright lanes enclosing dark cells, with sizes of ~20,000 km in radiance images of emission lines (3). The network lanes (networks thereafter) are believed to be locations of strong magnetic fluxes originating from the boundaries of convection cells with similar sizes in the photosphere. Previous observations of coronal holes with the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) instrument (4) onboard the Solar and Heliospheric Observatory (SOHO) revealed Doppler blue shifts of 5-10 km s-1 for emission lines formed in the upper TR (5). They were interpreted as signatures of the nascent solar wind guided by funnel-like magnetic structures originating from the networks (6). Recent analyses revealed weak blue wing enhancements in profiles of emission lines formed in the TR (7, 8). These weak enhancements indicate the possible presence of a plasma component flowing upward with speeds of 50-100 km s-1, which may provide heated mass to the solar wind (8). It has been difficult to test this proposed idea without direct imaging of such TR upflows on the solar disk. However, moderate-resolution observations have revealed signatures of chromospheric upflows being heated to TR temperatures at the solar limb in a coronal hole (9). Using observations from the Interface Region Imaging Spectrograph (IRIS) (10), we report results from direct imaging on the solar disk of high-speed upflows with apparent speeds of 80- 250 km s-1. Thanks to the high resolution (~250 km) in new wavelength windows, IRIS slit-jaw imaging observations with the 1400Å, 1330Å, and 2796Å filters (see Supplementary Materials, SM thereafter) unambiguously reveal the prevalence of small-scale jet-like emission features from the bright networks (Figs. S1-S3, movies S1-S2). These three filters sample emission from the Si IV, C II and Mg II ions which are formed at temperatures of ~105 K, ~3×104 K and ~104 K, respectively. These network jets usually show fast upward motion with no obvious downward component. Although these jets are more easily seen in coronal holes located near the solar limb (movies S1-S5), they are clearly detected at any location on the solar disk outside active regions (movie S6). These network jets are best seen in 1330Å images. The jet widths are usually around ~300 km and approach the instrument resolution limit, suggesting that the actual widths of many jets may be even smaller. By applying the space-time technique (SM) to the 1330Å image sequence obtained on 23 January 2014 (Table S1, movie S2), we have quantified the apparent speeds and lifetimes for 63 randomly selected jets (Fig. 1). The speeds fall mostly in the range of 80-250 km s-1, which is much larger than the sound speed and close to the Alfvén speed in the chromosphere (11) and TR. These velocities are significantly larger than previously reported jet velocities in the chromosphere and TR (12-16). Some jets also show signatures of acceleration. Their lifetimes range mainly from 20 to 80 seconds. Most jets extend to lengths of 4-10 Mm (1 Mm = 106 m), although some clearly reach ~15 Mm. Many network jets also exhibit obvious motions transverse to their propagation direction, indicating that they carry transverse magneto-hydrodynamic waves known as Alfvén waves (11, 17). The wave magnitudes are difficult to measure from slit-jaw images because strong emission from other features complicates the quantification of the transverse displacement, and the jet lifetimes are usually too short to allow the detection of a full wave cycle. Instead, we use 2 spectroscopic observations to estimate the approximate velocity amplitudes of Alfvén waves. The root-mean-square value of the fluctuating Doppler shift of the Si IV 1393.77Å line is ~5 km s-1, which can be regarded as the resolved wave amplitude (SM, Fig. S5). Many of these network jets are likely the on-disk counterparts and TR manifestation of type-II spicules (SM), which are jet-like features moving upward with speeds of 50-110 km s-1 in the chromosphere above the solar limb (15, 16). Our direct imaging of flows along these jets on the solar disk is almost unaffected by line-of-sight superposition, thus providing further support for the debated existence of high-speed jet-like features (16, 18). IRIS observations also reveal their origin in the networks, which off-limb observations cannot determine. Yet, we notice that network jet velocities are generally twice that of type-II spicules, suggesting that the network jets sampled by the TR passbands are those being heated and accelerated in the upper chromosphere and TR (19), and/or that the apparent speeds we observe here are not all caused by mass flows. Additional absorbing components at the blue wings of some chromospheric absorption lines were previously claimed to be on-disk counterparts of type-II spicules (13). These features with speeds of 20-50 km s-1 are likely the lower-temperature parts and/or less-accelerated phase of the network jets. Many network jets tend to recur at roughly the same locations on timescales of ~2-15 minutes. Our on-disk observations show that these jets originate from localized bright regions in the networks (Fig. 2, movie S4). Sometimes we see obvious brightening at the footpoints of these jets. A few jets appear to reveal the characteristic inverted “Y”-shape morphology (Fig. 2B) that is associated with a bipolar magnetic field line reconnecting with a unipolar large-scale field (12). These characteristics, together with the high speeds, suggest that some of these intermittent jets may result from repeated magnetic reconnection (20) between small magnetic loops and the background open flux in the networks. It is also possible that the source regions of these jets are too small to be resolved by IRIS, or that other mechanisms (SM) such as flux emergence and the associated Lorentz force are responsible for the acceleration of the jets (21). Spectroscopic observations from IRIS reveal that many jets reach temperatures of at least ~105 K, the formation temperature of the Si IV 1393.77Å line under ionization equilibrium. The most prominent signature of network jets in Si IV line profiles is a significant increase of the line broadening, which could be a consequence of field-aligned flows (22) or unresolved transverse motions such as Alfvén waves (23) and twists (24). Combined imaging and spectral observations of IRIS can help evaluate the contribution from field-aligned flows and transverse motions. Greatly enhanced widths of the Si IV line are found around two locations of network jets (Fig. 3). The slit crosses the lower part of a recurring jet complex at location 1.